The invention relates to a hydraulic system for a working machine aid to a working machine comprising such a hydraulic system.
The invention is applicable on working machines within the fields of industrial construction machines, in particular wheel loaders, articulated haulers and excavators. Although the invention will be described hereinafter with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other heavy working machines, such as dump trucks, or other construction equipment.
Hydraulic systems are used in a wide range of applications. For example, working machines rely on hydraulic systems to provide power for handling a load. A hydraulic system for a working machine may comprise both linear actuators, i.e., hydraulic cylinders and rotary hydraulic machines, such as for example a hydraulic motor.
As the demands for energy efficiency in working machines are increasing, it is becoming more and more important to ensure that all parts of the working machine is as energy efficient as possible. Even though the energy efficiency of individual components in a general hydraulic system has been improved, the overall system efficiency tends to be low, and it may even be below 10%. Energy losses in the hydraulic system thus lead to increased fuel consumption, increased emissions and also to the need for additional cooling systems.
A hydraulic system may be used to power both linear actuators such as a hydraulic cylinder and rotating actuators such as a hydraulic motor.
Hydraulic motors may for example be used to provide the swing function of an excavator, as a fan motor of a wheel loader, hub-unit of a wheel drive, or for the track-drive of a crawler excavator.
A rotating hydraulic motor produces it mechanical torque T given a certain machine displacement D and a difference pressure ΔP. Accordingly, the output torque of the hydraulic motor can be controlled by controlling either the displacement D or the pressure ΔP. Furthermore, there is often a limitation of the maximum how within the supply system, which in turn gives rise to a maximum achievable rpm of the hydraulic motor.
In applications where a fixed displacement hydraulic motor is used, the torque is controlled by controlling the pressure, either by using a proportional valve or through some other pressure regulating means, such as a pressure regulating pump.
For a variable displacement hydraulic motor, it is also possible to control the output torque by varying the displacement. This enables the use of a hydraulic supply system providing constant pressure, which may be advantageous for example if it desirable to include a hydraulic energy storage in the hydraulic supply system.
In many applications, it is desirable that the hydraulic system is able to provide a continuously variable power output, in order to ensure smooth operation for varying loads. A continuously variable output may for example be achieved by using a variable displacement hydraulic machine as outlined above. However, as a variable displacement hydraulic machine is more complicated and thus more expensive than a fixed displacement hydraulic machine, for certain applications it may be more advantageous to use a fixed displacement hydraulic machine and where the variation of the power output is controlled by either by controlling the pressure or the fluid flow to the hydraulic machine.
To achieve a more energy efficient and flexible hydraulic system, the output torque of a constant pressure system can be controlled by connecting a proportional servo valve to a fixed displacement hydraulic machine.
However a constant pressure system using a proportional servo valve is only fully energy efficient if used at full load, i.e. when the valve is fully open. For partial loads, losses may occur in the servo valve, the lower the load, the higher the losses.
Accordingly, it is desirable to provide a more energy efficient hydraulic system.
In view of the above-mentioned desired properties of a hydraulic system for a working machine, and the above-mentioned and other drawbacks of the prior art, it is desirable to provide an improved hydraulic system for a working machine.
According to a first aspect of the present invention, it is therefore provided a hydraulic system comprising: a first rotating hydraulic machine and a second rotating hydraulic machine, said first and second hydraulic machine being arranged to provide a torque via a common output shaft; a first valve means for providing a differential hydraulic pressure level over the first hydraulic, machine by using two sources of hydraulic fluid having different hydraulic pressure levels, a second valve means for providing a differential hydraulic pressure level over the second hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels, a control unit configured to control said first valve means and said second valve means such that different discrete levels of torque are provided via the output shaft of the hydraulic system.
The present invention is based on the realization that different discrete levels of torque can be provided by applying a differential pressure over two separate hydraulic machines which are connected to the same output shaft.
According to one embodiment of the invention, at least one of the first and second hydraulic machines may be a fixed displacement hydraulic machine. Hydraulic machines having a fixed displacement are in general less complex compared to variable displacement hydraulic machines and can therefore be made at a lower cost. In a fixed displacement hydraulic machine, the output torque is controlled by controlling the differential pressure over the hydraulic machine.
In one embodiment of the invention, the first hydraulic, machine may have a first fixed displacement and the second hydraulic machine may have a second fixed displacement, the second fixed displacement being the same as the first fixed displacement.
Furthermore, in one embodiment of the invention, the second fixed displacement may be different from the first fixed displacement. By using only fixed displacement hydraulic machines, an energy efficient power system can be achieved. Furthermore, by using hydraulic machines having different fixed displacement connected to a common output shaft, the number of discrete torque levels that can be provided is increased.
According to one embodiment of the invention, the first hydraulic machine may be a fixed displacement hydraulic machine and the second hydraulic machine may be a variable displacement hydraulic machine.
It may be advantageous to use a variable displacement hydraulic machine in combination with one or more fixed displacement hydraulic machines in order to be able to provide a continuously variable output torque.
In one embodiment of the invention, the differential hydraulic pressure level may be provided by using a first hydraulic accumulator having a first hydraulic pressure level and a second hydraulic accumulator having a second hydraulic pressure level, the first hydraulic pressure level being different from the second hydraulic pressure level. By using hydraulic accumulators having fixed pressure levels for providing hydraulic fluid to the hydraulic machines via valve means, energy may be recuperated from the output shaft, via the hydraulic machines and stored in the accumulator, thereby increasing the energy efficiency of the hydraulic system. In embodiments where two or hydraulic accumulators are used in combination with a variable displacement hydraulic machine, it may be sufficient that only the hydraulic accumulator having the highest pressure is connected to the variable displacement hydraulic machine, and the low pressure side of the variable displacement hydraulic machine may then be connected to the accumulator having the lowest pressure, or to a tank representing the low pressure source. However, it may be advantageous to connect all hydraulic accumulators also to the variable displacement hydraulic machine in order to provide increased flexibility during operation of the hydraulic system. A further advantage of using one or more hydraulic accumulators in the power system is that it is possible to downsize the supply unit providing energy to the system, as such a supply unit can be dimensioned to handle the average energy requirement while one or more hydraulic accumulators can provide energy to accommodate peak loads.
In one embodiment of the invention, a pressure level of the first hydraulic accumulator may be fixed within a first predetermined range, and a pressure level of the second hydraulic accumulator may be fixed within a second predetermined range, the second predetermined range being different from the first predetermined range. The predetermined range may be small, such that the pressure level can be regarded as substantially constant, or the energy range may be relatively large such that different pressures can be provided by the hydraulic accumulators depending on different modes of operation of the hydraulic system. Furthermore by allowing the pressure of the accumulator to be varied, a particular hydraulic system may be used in applications having different requirements with regards to torque output.
Furthermore, the hydraulic system may advantageously comprise an accumulator charging system such as a hydraulic pump connected to an engine of a vehicle in which the hydraulic system is arranged. Through the hydraulic charging system, the pressure levels in the hydraulic accumulators may be maintained within predetermined pressure ranges. An additional advantage of using hydraulic accumulators for providing energy to the hydraulic system is that, for certain applications where allowable by the work cycle of the power system, the power supply, such as a combustion engine, may be reduced in size compared to if the power supply is directly connected to the hydraulic machines.
According to one embodiment of the invention, the first valve means may advantageously comprise a first valve connected between a first source of hydraulic fluid and a first side of the first hydraulic machine and a second valve connected between a second source of hydraulic fluid and a second side of the first hydraulic machine, and the second valve means may comprises a first valve connected between a first source of hydraulic fluid and a first side of the second hydraulic machine and a second valve connected between a second source of hydraulic fluid and a second side of the second hydraulic machine. The valves are arranged to control the flow between a source of fluid and a respective side of the hydraulic machine, so that a differential pressure may be provided over the hydraulic machine by controlling, the respective valves. In principle, the valve means may comprise a valve block, or a valve matrix, having a plurality of inlets and a plurality and a plurality of outlets, and being configured to provide the desire functionality.
According to one embodiment of the invention, the valve means may advantageously comprise on/off valves. Although conventional flow regulating valves may be used to provide the desired function, it is preferable to use on/off valves as they can have a more simple construction, and thereby be made at a lower cost than a proportional servo valve. It is also desirable to be able to switch the valve fast, which can be achieved by an on/off valve.
Furthermore, an on/off valve is more energy efficient in comparison to a proportional servo valve used proportionally. Thereby, the energy efficiency of the system may be further improved through the combination of hydraulic accumulators each having an essentially constant pressure level being connected to fixed displacement hydraulic machines via on/off valves. An on/off valve can for example be provided in the form of a poppet valve or a spool valve. An on/off valve is sometimes also referred to as a digital valve. In principle, it is preferable to use a valve having a relatively small or negligible pressure drop over the valve. The person skilled in the art readily realizes that various valve arrangements and types of valves are possible while still providing the desired function of controlling the differential pressures over the respective hydraulic machines.
In one embodiment of the invention, the first hydraulic machine may have a first fixed displacement D1 and the second hydraulic machine may have a second fixed displacement D2=D1*(2*n+1), where n is the number of different hydraulic, pressure levels available in said hydraulic system. It is advantageous to select the displacement of the fixed displacement hydraulic machine in relation to the number of available pressure levels in the system such that as many discrete torque levels as possible can be provided via the output shaft, and such that the difference in torque, i.e. the torque step, is the same for any two adjacent torque levels.
Furthermore, in a hydraulic system comprising k fixed displacement hydraulic machines, k≧2, each hydraulic machine having a fixed displacement different from a fixed displacement of any of the remaining hydraulic machines, and n different hydraulic pressure levels, n>2, the displacement for hydraulic machine k may advantageously be selected as Dk=D1*(2*n+1)k−1, where D<i is the fixed displacement of the hydraulic machine having the lowest fixed displacement.
According to one embodiment of the invention, the hydraulic system may advantageously comprise at least three different pressure levels, wherein a difference in pressure between the two lowest pressure levels, pi and p2, is Δρ1=p2−p1, and wherein a pressure of the nth pressure level is selected as pn=p1+η*Δp1. By selecting the n pressure levels (n>2) according to the aforementioned description, the resulting number of discrete torque steps nj for each hydraulic machine, of equal step size, is nt=1+(n−2)*2. Accordingly, the total number of torque steps is nT times the number of fixed displacement hydraulic machines having different displacement. The different pressure levels may for example be provided by hydraulic accumulators, or they may be provided by other means for providing a hydraulic pressure such as a hydraulic pump arrangement.
According to a second aspect of the invention, there is provided a hydraulic system comprising: a rotating hydraulic machine arranged to provide a torque via an output shaft; a first valve means for providing a first differential hydraulic pressure level over the hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels, a second valve means for providing a second differential hydraulic pressure level over the hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels, an absolute value of the second differential hydraulic pressure level being different from an absolute value of the first differential hydraulic pressure level; and a control unit configured to control the first valve means and the second valve means such that different discrete levels of torque are provided via the output shaft of the hydraulic system.
It is further realized by the inventors that it is possible to provide different discrete levels of torque by applying a different differential pressures over a hydraulic machine using discrete pressure levels to achieve the different differential pressures.
In one embodiment of the invention, the rotating hydraulic machine may advantageously have a fixed displacement. Thereby, the output torque is controlled through the different discrete differential pressures which may be applied over the fixed displacement hydraulic machine.
In one embodiment of the invention, the hydraulic system may advantageously comprise a first source of hydraulic fluid having a first hydraulic pressure level, a second source of hydraulic fluid having a second hydraulic pressure level different from the first pressure level, and a third source of hydraulic fluid having a third hydraulic pressure level different from the first and the second pressure level; wherein the first valve means and the second valve means use one source of hydraulic fluid in common. Through the use of three different sources of hydraulic fluid having different pressure levels, a plurality of different differential pressures can be applied over the fixed displacement hydraulic machine in order to provide different discrete levels of output torque on the output shaft.
Furthermore a difference in pressure between the second pressure level and the third pressure level is advantageously substantially twice the difference between the first pressure level and the second pressure level. By selecting the difference between the highest and the middle of the pressure levels to be twice the size of the difference between the lowest pressure level and the middle pressure level, an optimal number of equidistant levels of output torque can be provided.
Further effects and features of this second aspect of the present invention are largely analogous to those described above in connection with the first aspect of the invention.
There is also provided a working machine comprising a hydraulic system according to any one of the aforementioned embodiments.
According to a third aspect of the present invention, there is provided a method for controlling a hydraulic system to provide discrete levels of output torque, the system comprising: a plurality of hydraulic machines arranged to provide a torque via a common output shaft, and valve means for providing a plurality of differential hydraulic pressures to each of the hydraulic machines, via the valve means from a plurality of sources of hydraulic fluid; the method comprising: controlling the valve means in response to a requested output torque level such that discrete output torque levels are provided by the output shaft by: if the requested output torque level is equal to or lower than a minimum torque level of the hydraulic system, providing the minimum output torque level by applying the lowest differential hydraulic, pressure to a hydraulic machine having the lowest fixed displacement; if the requested output torque level is equal to or higher than a maximum torque level of the hydraulic system providing a maximum output torque level by applying the highest differential hydraulic pressure to all of the plurality of hydraulic machines; and if the requested output torque level is between the minimum output torque level and the maximum output torque level, applying a differential hydraulic, pressure to at least one of the hydraulic machines such that a torque level closest to the requested torque level is provided.
According to a fourth aspect of the present invention, there is provided a method for controlling a hydraulic system to provide discrete levels of output torque, the system comprising: a hydraulic machine arranged to provide a torque via an output shaft, and valve means for providing at least three differential hydraulic pressures to each of the hydraulic machines, via the valve means, from a plurality of sources of hydraulic fluid; the method comprising: controlling the valve means in response to a requested output torque level such that discrete output torque levels are provided by the output shaft by if the requested output torque level is equal to or lower than a minimum torque level of the hydraulic system, providing the minimum output torque level by applying the lowest differential hydraulic pressure to the hydraulic machine; if the requested output torque level is equal to or higher than a maximum torque level of the hydraulic system, providing a maximum output torque level by applying the highest differential hydraulic pressure to the hydraulic machine; and if the requested output torque level is between the minimum output torque level and the maximum output torque level, applying a differential hydraulic pressure to the hydraulic machine such that a torque level closest to the requested torque level is provided.
According to a fifth aspect of the present invention, there is provided a method for controlling a hydraulic system to provide discrete levels of output torque, the system comprising: a first hydraulic machine having a fixed displacement, arranged to provide a torque via a common output shaft; a second hydraulic machine having a variable displacement, arranged to provide a torque via the common output shaft, a first valve means for providing a differential hydraulic pressure level over the first hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels; a second valve means for providing a differential hydraulic pressure level over the second hydraulic machine by using two sources of hydraulic fluid having different hydraulic pressure levels; the method comprising the steps of: controlling the valve means and the second hydraulic machine in response to a requested output torque level such that discrete output torque levels are provided by the output shaft by: applying a differential hydraulic pressure to the first hydraulic machine such that a torque level closest to the requested torque level is provided; and controlling the displacement and direction of the second hydraulic machine to add torque to or receive torque from the output shaft such that the requested output torque is provided.
Effects and features of the third, fourth and fifth aspects of the present invention are largely analogous to those described above in connection with the first and second aspect of the invention.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.